The present invention relates to the field of medical monitoring instrumentation in general, and in particular, to the field of labor monitoring devices.
It is well known that the process of labor and childbirth entails the gradual thinning and dilation of the uterine cervix, in response to coordinated uterine contractions, resulting in the eventual expulsion of the fetus through the fully dilated uterine cervix and birth canal. As such, the degree, and rate of progression, of cervical dilation is closely monitored by hospital staff during labor and childbirth, and is considered to be the cardinal indicator of the progression of labor. Slow or inadequate cervical dilation may indicate inefficient or pathological labor, and is often an indication for medical or surgical intervention, particularly if accompanied by evidence of fetal distress. In addition, the gradual descent of the fetal head through the birth canal, also referred to as the descent of the "presenting part", is repeatedly assessed during labor, and serves as an additional descriptor of the progress of childbirth.
Cervical dilation does not ordinarily proceed at a constant rate. Typically, dilation is slow until 4-5 cm dilation is reached, more rapid until dilation is nearly complete, and then slower again until full dilation of 10 centimeters is achieved. Thereafter, the second stage of labor, during which the fetus passes through the dilated cervix and birth canal, begins.
Due to their cardinal importance as indicators of the progress of labor, the amount of cervical dilation and the descent of the presenting part are regularly determined by professional attendants (such as midwives and obstetricians) during the course of labor, usually by means of a manual vaginal examination. On average, ten manual vaginal examinations are required per labor. Manual vaginal examinations, however, suffer from several disadvantages: they are embarrassing and uncomfortable for the patient, subjective, inaccurate, provide very intermittent information, can introduce infection into the uterus, require manual charting, and add to the medical staff workload.
Although several monitoring devices are routinely used during the course of labor (such as fetal heart rate monitors, fetal oxygen saturation monitors [pulse oxymetry], uterine activity monitors [tocometry], and maternal vital signs monitors), automatic monitoring of cervical dilation and descent of the presenting part is not yet available. This is despite the fact that several methods for measuring cervical dilation have been described. These methods include:
1) Obstetric gloves incorporating a measuring string or measuring tape. PA1 2) Finger mounted angular V calipers. PA1 3) Cervix mounted angular V calipers. PA1 4) Induction transmitters and receivers clamped to two sides of the cervix opposite to each other. PA1 5) A multi-switch membrane that is inserted into the uterus, and pressed between the cervical internal os and the fetal head.
This device is attached to the obstetricians fingers, and is inserted into the vagina whenever a measurement is required. The obstetrician measures dilation by spreading his fingers in the sane way as done in regular manual vaginal examination. A mechanical scale, potentiometer, or strain gauge measures the angle between the caliper alms, and the measurement is converted to a dilation value. Both of the above devices suffer from the deficiencies described above for standard manual vaginal examinations. PA2 This device is placed in the vagina and attached to two points on opposite sides of the cervix to each other. A mechanical scale, potentiometer, or strain gauge then measures the angle between the caliper arms, and the measurement is converted to a dilation value. This device suffers from the deficiency that it substantially occupies the vagina, thus interfering with other monitoring and treatment activities. It therefore has to be removed and reinserted repeatedly. In addition, it is uncomfortable to the patient, may require manual charting, and is difficult to install, thus adding to the medical staff workload. PA2 As the distance between the primary and secondary induction windings affects the induced signal, the distance between them (which is equal to the cervical diameter) can be measured. This device suffers from the deficiency that it functions effectively only until about 5-7 cm dilation, whereas manual measurements are required until 10 cm dilation. In addition, the insertion of other instruments into the vagina and the cervix, as is often done during labor, adversely effects the measurement readings of the device, rendering it impractical for clinical use. PA2 As the switches in the cervical opening are not pressed, while those within the uterus are, an ongoing indication of the progress of dilation is rendered. This device suffers from several deficiencies. Firstly, it is necessary to insert the membrane between the fetus and the uterine wall, which is technically difficult, particularly before the amniotic sack has ruptured, and undesirable afterwards due to the risk of introducing infection. Secondly, the membrane often shifts, giving rise to false measurements. In addition, as the membrane crosses the cervix from one side to the other after placement, the entrance to the uterus is obstructed, thus interfering with other monitoring and treatment activities. Other disadvantages are that the device is uncomfortable for the patient, and adds to the medical staff workload.
None of the devices described above are able to monitor descent of the presenting part.
It is well known that it is possible to accurately and continuously measure the degree of dilation of a hollow organ by means of ultrasound tissue imaging. Successfully measuring cervical dilation by this technique would obviate the deficiencies and drawbacks inherent to the alternative techniques described above. As such, there has been much interest in developing ultrasound-based cervical dilation monitors. All such devices described to date incorporate two transducers, one being an ultrasound transmitter and the other an ultrasound receiver, which are attached to the cervix. The time taken for ultrasound waves transmitted from the transmitter to reach the receiver is translated into the distance between the two transducers, which represents the diameter of the cervix. Devices of this nature have been shown to operate satisfactorily under well-controlled laboratory conditions. However, in-vivo, the presence of air and differing biological tissues within the vagina creates an inhomogeneous medium between the two transducers. This lowers the signal-to-noise ratio and results in inaccurate ultrasound measurements and low repeatability. Furthermore, the probe inserted into the vagina, being a complete ultrasound transmitter, is relatively large (and thus must be removed to allow other monitoring and treatment activities), and may be uncomfortable for the patient. In addition, as mentioned above regarding non-ultrasound based labor monitors, devices of this nature do not monitor descent of the presenting part.
There is therefore a need for an ultrasound-based cervical dilation monitor which is capable of accurately and automatically monitoring the progress of cervical dilation and decent of the presenting part, without the need to introduce a large ultrasound probe into the vagina.